Electric power steering apparatus of the type concerned are known and used heretofore. In one such known apparatus, which is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2005-16617, a proposal has been made to enhance the wear durability of a worm gear mechanism by improving the lubricating performance using a worm gear and a worm wheel gear having a tooth form or profile in a special form, which is different from an involute form and can retain most part of a lubricating agent (grease) on tooth surfaces within the tooth width of the worm wheel gear. Due to the specially shaped tooth form employed, the worm gear and worm wheel gear are relatively uneasy to manufacture and require a relatively high manufacturing cost.
In general, the worm gear and the worm wheel gear have two or three gear teeth working together on a working line. Not all the working points where the worm gear teeth and the worm wheel gear teeth are meshing together does lie on the pitch circle of the worm wheel gear. At one working point, the addendum of the worm gear tooth and the dedendum of the worm wheel gear tooth are working. In this instance, the working point is offset from the pitch circle toward the dedendum of the worm wheel gear and, hence, the load is concentrated at the dedendum of the worm wheel gear. This demands adequate consideration of the wear durability of the gear teeth.
Furthermore, as for the worm gear, since the working point is offset from the pitch circle toward its addendum side, the lead angle is smaller than that achieved in the case where the working point lies on the pitch circle. The small worm gear lead angle reduces the worm gear transfer efficiency and reversing efficiency, which gives a negative influence on the steering feel.
In the worm gear mechanisms, in order to ensure smooth meshing and low-noise operation, it may be considered that the worm gear is made of metal and the worm wheel gear is formed of a synthetic resin material. In this case, however, since the stiffness of the plastic worm wheel gear is relatively low, the worm wheel gear is likely to cause a creep strain due to heat generated during working together with the worm gear. Accordingly, a consideration should be made to the durability of the worm wheel gear.